In Washington's Crossing, what made the Battle of Trenton a bloody but decisive American victory?

The key to Washington's success at the Battle of Trenton
in December of 1776 was surprise.  No one could have expected Washington's beaten,
shrinking and demoralized army to be able to successfully cross the river--onto the same
side as the massive 20,000 strong force under General Howe--and march on a force of
experienced, tough mercenaries such as the Hessians guarding the
city.

It was Christmas morning, and many of the Hessians
were hung over.  This was their day off, they had celebrated the night before, and they
were supremely overconfident.  This was garrison duty after all, and they were only
expected to watch the colonials across the river so that they did not escape while Howe
waited for the river to freeze.

This is what makes
Washington's attack so brilliant.  He had no alternative, he went against every rule of
military training he had received and took the Hessians completely by surprise.  What
made it bloody was that much of the fighting was hand to hand combat, house to house,
where bayonets and pikes were used in place of rain-soaked
gunpowder.